In amorphous silicon TFT (thin film transistor) forming processes included in an LCD (liquid crystal display) fabrication, an etching process must be carried out for plural times. Thus, with the conventional art, a photolithography process including an exposing process and a developing process is carried out for plural times corresponding to the number of the etching processes. Different coating-and-developing apparatuses and exposure apparatuses are required for forming different patterns for respective etching processes in a TFT forming process, and the total system cost is thus expensive.
As one solution of the above problem, a reflow process, which dissolves and deforms a resist film of a first pattern once used as an etch mask to reshape the same into a new, second pattern, attracts attention in these days. With the use of the reflow process, it is not necessary for forming the second resist pattern to perform processes employing a coating-and-developing apparatus and an exposure apparatus. The reflow process not only reduces the total system cost but also improves the production efficiency.
A series of processes for formation of an amorphous silicon TFT including a reflow process will be described with reference to FIG. 10. As shown in FIG. 6(a), on a gate electrode 201 formed on a glass substrate 200, an insulating layer 202, an Si-layer 203 composed of an a-Si layer (i.e., non-doped amorphous Si layer) 203a and an n+a-Si layer (i.e., phosphor-doped amorphous Si layer) 203b, and a metal layer 205 for forming drain and source electrodes are stacked in that order.
Then, in order to etch the metal layer 205, by a photolithography process, a resist film 206 is formed on the metal layer 205 and a first pattern is formed in the resist film 206 through an exposing step and a developing step. A half exposing technique is used in the exposing step so as to obtain the resist film 206 having thickness distribution (i.e., the resist film 206 has thick portions and thin portions.). The half exposing technique employs a half-tone mask having light-transmittance distribution. The half exposing technique is described in U.S.2004126713A1 (JP2005-108904A), for example. The resist film 206 having the first pattern is used as an etch mask for etching the metal layer 205, and portions of the metal layer 205 which are not covered with the mask are etched and removed, as shown in FIG. 6(b).
Altered layer 207 is formed in the surface region the resist film 206 due to application of a wet etching liquid used for etching the metal layer 205. As a pre-treatment of a reflow process, the altered layer 207 is removed by supplying thereto an alkaline solution, as shown in FIG. 6(c).
Then, as shown in FIG. 6(d), portions of the resist film 206 which are not necessary for a second etching process (i.e., the thin portions of the resist film 206) are removed by re-developing process, while portions of the resist film 206 near targets Tg (i.e., the thick portions of the resist film 206) remain.
Then, the remaining resist film 206 as shown in FIG. 6(d) is exposed to a solvent vapor-containing atmosphere. Thereby, the resist film 206 dissolves and diffuses (i.e., reflow) to move onto the targets Tg to cover the same. Thus, the resist film 206 is reshaped into a second pattern, in other words, a second resist pattern is formed. Then, the Si layer 203 is etched by using the metal layer 205 and the resist film 206 as masks, as shown in FIG. 7(a); and the resist film 206 is removed, as shown in FIG. 7(b). Thereafter, the n+a-Si layer 203 in channel regions is etched so that a TFT structure is formed, as shown in FIG. 7(c).
In the foregoing reflow process, the substrate is placed on the temperature adjusting plate whose temperature is set to a predetermined constant value, and the substrate whose temperature is adjusted by the temperature adjusting plate is exposed to a solvent atmosphere such as a thinner gas atmosphere that dissolves the photoresist and reshapes it into a new pattern.
The foregoing reflow process, however, has a problem that it is difficult to appropriately reflow the resist film. That is, depending on the process conditions of the reflow process, the resist film, in some cases, reflows excessively to spread wider than desired so that the targets to be masked are not sufficiently covered by the resist film, or the resist film, in some cases, reflows slowly so that sufficient process efficiency can not be achieved.